The majority of point-to-multipoint radio communications systems operate at relatively high bandwidths, due to the high data rates and real-time requirements associated with data receipt and transmission. It would be attractive to operate at low bandwidths for applications having less stringent data rate requirements because of the commensurate advantages in relation to range and reduction in power requirements. However, low bandwidth systems can incur significant frequency lock problems due to the fact that the frequency error between the transmitter and the receiver can be much greater than the signal bandwidth; the identification of the frequency error typically involves use of high accuracy components, which equates to a significant overhead in terms of costs, and to a commensurate limitation in the use of low bandwidth systems.
U.S. Pat. No. 6,522,698 offers a low cost solution in which the bulk of the decoding and processing is performed in the central station, any given remote station simply having to transmit at a relatively low data rate: the remote station (or outstation) is configured so as to generate uplink messages arbitrarily in time, leaving it to the base station to identify the unique signature of any given remote station. Typically this involves providing the base station with many sliding detectors, which are expensive in terms of computational requirements, and, for a large number of remote stations, can become prohibitively costly.
It would be desirable to provide a lower cost narrow band transmission system that is suitable for use with a significant number of remote outstations.